This invention relates to an apparatus for electrically contacting the human body and more particularly to grounding devices that inhibit electro-static discharge from the human body.
Semiconductor devices constructed from metal oxide semiconductor field effect transistors ("MOS FET") are easily damaged by electro-static discharge. Examples of two well known semiconductor technologies that utilize MOSFET transistors are the "N" channel MOS ("NMOS") technology and the complimentary MOS ("CMOS") technology.
Each of the transistors that make up the semiconductor chip include an "N" or "P" type channel, a thin insulating oxide layer over the channel, and a conductive gate of metal or silicon over the insulating layer. One end of the channel is referred to as the "source" and the other end as the "drain". If a high voltage, such as is present in an electro-static discharge, comes into contact with the gate of the transistor, the thin oxide layer breaks down and current flows into the gate resulting in permanent damage to the transistor.
To make the transistors less susceptible to damage by electro-static discharge, a pair of diodes are normally used on each transistor to clamp the gate voltage between set limits. In particular, the anode of a first diode is connected to the transistor's gate and the cathode is connected to the integrated circuit's positive power supply terminal, while the cathode of a second diode is connected to the gate and the anode is connected to the I.C.'s negative power supply terminal. If a high voltage from an electro-static charge comes into contact with the gate causing the gate voltage to exceed the positive supply voltage, the first diode conducts and dissipates the electro-static charge. Similarly, if the gate voltage goes below the negative supply voltage, the second diode conducts and dissipates the electro-static charge. Although the diodes reduce the number of semiconductor chips that are destroyed, they do not completely eliminate electro-static discharge damage.
The major cause of electro-static discharge damage is the build up of electro-static charge on the human body and its subsequent discharge through the semiconductor during handling. Some clothing materials worn by those handling the semiconductors and the materials covering the floors that they walk on increase the amount of electro-static charge accumulated on the body, thereby increasing the potential for damage. Low relative humidity also compounds the problem, and voltage potentials on an electro-statically charged human body have been measured in the range of 4 to 15 Kilo-Volts.
To reduce or eliminate this accumulated electro-static charge on the human body, persons handling electro-static sensitive semiconductors usually wear a grounded wrist strap. Two types are commonly used. The first type is an elastic wrist strap which incorporates a non-conductive elastic band with a conductive foil attached to the inside of the elastic band. The elastic strap is intended to be worn on the wrist with the foil contacting the upper side of the wrist. The foil is secured to the elastic band with a rivoted snap and a grounding cable with a mating snap attaches to the foil at the rivoted snap.
There are several problems with the elastic strap. One problem is that permanent stretching and loss of elasticity occurs with use and the elastic band fails to hold the conductive foil against the wrist. Another problem occurs if the user wears the elastic strap with the conductive foil on the underside of the wrist. Worn in this fashion, the cable has a tendency to pull the foil away from the skin. When the elastic strap is worn with the foil on top of the wrist, however, the amount of hair that normally grows in that area of the wrist often prevents good electrical contact between the foil and the skin. In addition, the snap type connectors often become intermittant.
The second type of wrist strap is the metal wrist strap which incorporates a stretchable and conductive metal band, such as the type commonly used with wrist watches. A metal snap is attached to the metal band and a grounding cable with a mating snap attaches to the metal band at the snap. The metal band can be uncomfortable to wear because it often pinches the skin or pulls the hair of the wearer. When new, the band is sized to provide good electrical contact when worn on a small wrist. Consequently, blood circulation is somewhat restricted and discomfort occurs when the band is worn for an extended time on a larger wrist. When the band is alternately worn on different size wrists, as often occurs do to changes of personnel during different factory shifts, permanent stretching occurs with use and good electrical contact is no longer provided to a small wrist. As with the elastic strap, the snap type connector often becomes intermittant.